Two processes for production of phosphoric acid are practiced. One is the so-called "dry process" in which "yellow phosphorous" (white phosphorous containing some of its allotrophic red form) is burned in air to produce phosphorous pentoxide which is then hydrolyzed to phosphoric acid. (Yellow phosphorous itself is produced in an electric furnace by reduction of phosphate compounds contained in the ore heated to high temperature by electrical current between two electrodes.) The process is capable of producing high purity phosphoric acid called "technical grade". "Food grade" phosphoric acid, used for example in the beverage industry, is made by removing the small amounts of arsenic in the technical grade acid.
However, about 60% of the current total phosphoric acid production in the United States is from the "wet process". Analysts project total phosphoric acid production in the United States at nearly 8.5 million tons (in terms of phosphoric anhydride--P.sub.2 O.sub.5) for 1983 (Chem. & Eng. News, March, 1983, p.3). The fertilizer industry uses 85% of the "agriculture", "fertilizer", or "merchant" grade phosphoric acid made by the wet process. A major derivative for the agricultural industry is diammonium phosphate for fertilizer and other agricultural uses.
Wet process phosphoric acid is commonly manufactured by decomposing phosphate rock (apatite) with sulfuric acid. Any complex phosphate compound may be combined with either sulfuric acid or hydrochloric acid to obtain the wet process phosphoric acid. The priciple reaction is EQU Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 +10H.sub.2 SO.sub.4, 6H.sub.3 PO.sub.4 +10CaSO.sub.4 +2HF.
The calcium sulfate (gypsum) and other solids are generally removed by filtration. Even so, the resulting acid contains considerable impurities, the extent and type depend mainly upon the grade of rock used. The balance of impurities consists of various inorganic compounds as well as colloidal carbonaceous matter which give the acid a dark brown appearance. Typical analyses of apatite rock from two sources in the United States are shown in TABLE 1A with percentage of wet process phosphoric acid produced from each of them by treatment of the rock with a mixture of sulfuric acid and wet-process phosphoric acid containing 25% P.sub.2 O.sub.5.
TABLE 1A ______________________________________ Properties of Industrial Phosphoric Acids Wet Process (agricultural grade).sup.a Florida North Carolina Constituent, % Rock 43% H.sub.3 PO.sub.4 % Rock 37% H.sub.3 PO.sub.4 ______________________________________ P.sub.2 O.sub.5 34.3 31.0 32.9 26.9 CaO 49.5 0.13 54.0 0.10 MgO 0.25 0.30 0.27 0.52 Fe.sub.2 O.sub.3 1.33 1.18 0.80 2.09 Al.sub.2 O.sub.3 1.28 0.66 0.80 1.40 F 3.84 1.91 3.99 1.30 ______________________________________ .sup.a G. L. Bridger, et al., Ind. Eng. Chem. Process Des. Dev., 20 (3), 416-424, 1981.
In sharp contrast to the acid produced by the wet process, the clear, almost colorless phosphoric acid from the dry process (given in TABLE 1B) is very pure.
TABLE 1B ______________________________________ Food Grade Dry Process Acid.sup.a Constituent 75.1% H.sub.3 PO.sub.4 ______________________________________ P.sub.2 O.sub.5 54.4% H.sub.2 S &lt;0.1 ppm As.sub.2 O.sub.3 0.1 ppm Fe &lt;10 ppm Cl &lt;10 ppm F &lt;1 ppm NO.sub.3 &lt;5 ppm Heavy Metals &lt;5 ppm ______________________________________ .sup.a Anon., Monsanto Company Tech. Bull. 1C/DP239R
Food grade phosphoric acid is made from technical grade phosphoric acid (the product of the dry process) by the use of hydrogen sulfide gas bubbled into the acid to precipitate arseneous sulfide which is removed by filtration. (Some arsenic is always associated with phosphorous because of its similar chemical properties.)
Thus, wet process agricultural grade phosphoric acid contains considerably more impurities compared to dry acid. However, the dry process requires much more energy (electric furnacing at high temperatures) and is polluting to the atmosphere. Unprotected workers exposed to the elemental phosphorous vapors in the dry process may develop jaw bond decay.
In the United States the price of furnace (dry) phosphoric acid is currently about four times that of agricultural grade phosphoric acid. For these reasons, more attention in recent years has been directed toward the purification and upgrading of agricultural grade phosphoric acid to technical grade phosphoric acid which can then be readily converted to food grade phosphoric acid, especially since the reserves of high grade phosphate rock are being rapidly depleted in the United States.
Ion-exchange resins can be used to improve the purity of the agricultural grade phosphoric acid but cost and operating expense from inherent fouling of the resin make this process economically unattractive.
Much work has been done in recent years in the area of organic solvent extraction of phosphoric acid from wet-process water. High purity acid results since the organic solvent rejects most of the common impurities found in wet-process phosphoric acid. A literature review of the processes based on this principle is given by J. F. McCullough, Chem. Eng. 1976, 83 (26), 101-3. Other literature includes G. L. Bridger, Ind. Eng. Chem. Process Des. Dev., 1981, 20, 416-24; N. Takahara, Chem. Economy and Eng. Rev., April 1976, 8 (No. 4), 31-44; and M. S. Burova, et al., J. of App. Chem. of the U.S.S.R., Aug. 1978, 51 No. 8, Part 1, 1600-1604, as well as in U.S. Pat. No. 3,917,805. However, to date, processes based on the principle of solvent extraction are very complicated and require multiple extraction stages, high capital cost, and loss of solvent. In addition, the solvent which must be used is itself generally hazardous.
Active carbon has also been used to partly purify wet-process phosphoric acid. So far as known, its use has been restricted to removal of organic contaminants from the acid, and in processes which are different from the process of the present invention. Its use is described in U.S. Pat. Nos. 3,993,733; 3,993,736; 3,122,415 and in United Kingdom Patent No. 1,442,919. The active carbon purification requires a first step where the wet-process phosphoric acid is clarified, because it is easily fouled by suspended solids originally present in the acid after digestion of phosphate rock. In addition, the carbon is readily spent. A method by which a regeneration of the active carbon may be accomplished through further steps is described in European Patent Application No. 0,004,807.
Heretofore the purification by conventional means of crude wet-process phosphoric acid has only been accomplished by the use of high cost equipment and stringent controls. The operating costs of such a process make such a purification process only marginally profitable. Thus, there has been a great and long-standing need for a purification process requiring low capital and having low operating costs to "upgrade" the purity of wet-process phosphoric acid to the technical grade purity obtained with the prior art dry process.